POSTGLACIAL BIOTA OF THE GREAT LAKES REGION 127 



b. Small Lakes 



Many small lakes and streams have become filled up since the beginning 

 of post-Wisconsin time. The proof of the existence of these ancient lakes is 

 in the remains of mollusks and other animals which are now embedded in the 

 clay which fills them. In Iroquois County, 25 six miles northwest of Hoopston, a 

 mastodon was found in a bed of clay, associated with the following mollusks: 



Planorbis parvus Pisidium, resembling abditum 



Valvata tricarinala Valvata, resembling V. striata(=lewisii). 



Two miles southeast of Fairmount, 26 Vermilion County, the shells of Lym- 

 naea, Physa, Pianorbis, and Sphaerium occur in a light brown tenaceous clay 

 beneath one or two feet of black soil. A mastodon was secured from this 

 deposit; it was lying upon and partly embedded in the marly clay. The 

 location is in an old slough. 



On the Champaign till sheet, near the inner side of the Champaign moraine 

 several marl deposits occur which represent the bottom of old ponds which 

 filled kettle holes. Such deposits occur near the new greenhouses at the 

 University of Illinois, Urbana, Champaign County. The fauna, which is 

 quite extensive, indicates a climate colder than at present in this latitude, 

 several of the species now living far to the north of this locality, viz., Galba 

 obrussa decampi, Valvata sincera, Pisiidum tenuissimum calcareum, and Pisi- 

 dium costatum. Collected by Dr. T. E. Savage. 



Section of strata on the University Campus, Urbana 



4. Top soil or black clay without pebbles, grading into number 3 below 20-24 inches 



3. Clay, dark above, becoming light gray and more calcareous below; contain- 

 ing numerous shells of gastropods 18-20 inches 



2. Marl or limestone composed of more or less completely broken shells some- 

 what consolidated by cement of CaCO 3 8-12 inches 



1. Glacial till, pebbly, gray; the Champaign till sheet, early Wisconsin 6-12 inches 



The character of the shells and the position of the shell bed above the till 

 sheet leads to the inference that the pond may have contained the living shells 

 when the late Wisconsin ice was retreating in the northern part of the State. 

 The 20 species observed in this deposit are listed below: 



Sphaerium rhomboideum, rare 



" occidentale, rare 

 Musculium truncatum, common 



" rosaceum, rare 

 Pisidium adamsi affine, rare 



" cf. contortum, rare 



" costatum, common 



25 Collett, Dept. Geol. Nat. Res., Indiana, 2nd An. Rep., p. 386. 

 x Bradley, Geol. HI., TV, p. 242. 



